The touch screen technology has been widely used recent years in computing devices, such as mobile phones, notebook computers, and variety of portable electronic devices (such as game console, multimedia players, and the like). With this technology used in the user-interface, interaction between user and the computing device become more convenient and efficiency. Instead of using a mouse, keyboard, or any other intermediate devices, users can interact with what is displayed directly. Some touch screen can not only controlled through single or multi-touch gestures but also detect specially coated gloves and stylus.
Among a variety of different touch screen technologies, capacitive touch panels became more popular after the releases of new portable electronic devices. Unlike the surface capacitive touch panels can only detect single touch, the projected capacitive touch (PCT) panels are capable of implementing multi-finger touch detection. The PCT technology makes multi-gesture control possible like enlarge, narrow, rotate, or drag a pattern on a projected capacitive touch panel simultaneously.
Projected capacitive touch screens are made up of a matrix of rows and columns of resistive transparent conductive material such as indium tin oxide (ITO) layered on sheet of glass. The two axes of conductive material are driving electrodes and sensing electrodes. Capacitance exists among them.
As the human body is also an electrical conductor, touching the surface of the display results in a distortion of the screen's electrostatic field. There is a change in the level of capacitance. The chips measures cross capacitive in the X-axis and Y-axis of a projected capacitive touch screen structure and for every intersection of the drive/sense lines the capacity change is interpreted and converted to XY coordinates that correspond to the actual touch position.
Projected touch screen panel contains driving electrodes lay on X-axis direction and sensing electrode lay on Y-axis direction. In the common model of the operation, only one of the driving electrodes will connected to a periodic driving signal, the sensing electrodes can sense the capacitance change of the cross capacitive between driving electrodes and sensing electrodes. By scanning every sensing electrode, the capacitance change in each can be recorded. In order to finish a touch panel scanning, the total time of scanning of sensing electrodes is the number of driving electrodes times the number of sensing electrodes. In the implement of receiving sensing signals, one, or a plurality of ICs are needed to receive signals from sensing electrodes simultaneously for the sake of increasing the frame rate per second also called refresh rate.
The minimum frame rate is 50 frames per second. The higher frame rate the smoother the line drawing. It is easily understood that when drawing a line on screen, the start point is the first frame and the last point is the last frame. Filling more points between the start point and the last point can make the drawing line looking smoother. There is no standard for frame rate per second in specification, but the target frame rate in the current invention is 100 frames per second independent from touch panel size.
In the prior art, parallel processing is using to manipulate the data from sensing electrodes, which means one or several IC process signals from all sensing electrodes as the same time. Suppose there are M sensing electrodes, in this case, M analog circuit modules will needed to manipulate the data from each sensing electrode so that the demand of frame rate per second can be satisfied. This kind of IC architecture design has bigger die size and higher power consumption. It is not suitable for the large size touch application and driving electrodes with high resistance because of the high power consumption and the explosion of cost in order to satisfy the frame rate per second by adding more IC.
In the current invention, analog latches and multiplexers are added in the IC. The analog latches can cut the sensing signals received from sensing electrodes into a few pipes and the multiplexers can multi-tasking the sensing signals. The total sensing signal processing time can be shorten significantly. By multi-tasking the sensing signals, some circuits with typical function can be shared during non-critical pipe. The die area and power consumption can be saved in this case.